Optocoupler systems are useful for many different applications wherein electrical isolation is needed between a first circuit and a second circuit. One exemplary application is to use an optocoupler to electrically isolate a user interface (e.g., a logic interface) from a high voltage signal.
Optocoupler systems include a first circuit and a second circuit that are electrically isolated from each other. The first circuit includes a light emitting diode (LED) that is coupled to a LED current source. The first circuit is optically coupled to a second circuit. The second circuit includes a photodiode (PD). For example, the LED emits light, which impinges on the photodiode, causing a current through the photodiode (e.g., a photodiode current). The second circuit also includes a transimpedance amplifier circuit that is coupled to the photodiode to generate an output voltage signal that is based on the photodiode current. The second circuit also includes a current source that generates a reference current. Typically, the photodiode current is compared with the reference signal, and this comparison is utilized to generate the output voltage signal.
An optocoupler system includes three primary components: 1) buffer, 2) isolation, and 3) detector. The buffer provides a constant current to drive a light source (e.g., a light emitting diode (LED)). This current is referred to as the LED current and causes the LED to light up. The light passes through the isolation that can be, for example, a transparent substance. It is noted that the light undergoes a certain amount of attenuation before receiving the detector. This attenuation is one primary cause for the optocoupler system to have a current transfer ratio (CTR).
The detector converts the received light into corresponding electrical signals (e.g., a current signal and a voltage signal). The detector compares the voltage signal corresponding to the received light to a reference signal and generates an output signal (e.g., a “1” or “0”) that is based on the comparison.
However, the design of optocoupler systems poses significant challenges and introduces many issues and design concerns. One problem encountered in existing optocoupler systems is that the current transfer ratio of the optocoupler system changes with respect to temperature. In other words, the ratio between the photodiode current and the LED drive current is a function of temperature. One disadvantage of prior art optocouplers is that the LED drive current does not compensate for the effect of temperature variation on the current transfer ratio. To account for this variation, these optocouplers set a reference current to a lower value than would otherwise be set when the variation is not present. Consequently, the detector is more susceptible to ground noise, which is undesirable.
Based on the foregoing, there remains a need for a current transfer ratio temperature compensation method and apparatus that overcomes the disadvantages set forth previously.